Large aperture solar concentrating reflectors, usually having a cross-section which is substantially parabolic or spherical (generally called "dishes" because of the dish-like shape of the reflectors) have been under development for more than 20 years. Such collectors with large dishes have potentially the highest efficiency of conversion of incident solar energy into heat. They can be used to provide the highest useful temperatures.
For example, a solar collector installation using trough-like collectors of parabolic cross-section to supply heat to create steam to drive turbogenerators is reported to be currently supplying 354 megawatts of electrical power to the electricity grid in California, USA.
A recent design of a solar collector with a large aperture dish is described in the specification of International patent application No. PCT/AU93/00588, which was published as WIPO Publication No. WO 94/11918. That solar collector, which is illustrated in FIGS. 1 and 2 of the drawings of WIPO Publication No. WO 94/11918, is also featured in the specifications of Australian patent No. 677,257, Israel patent No. 107,647 and U.S. patent application Ser. No. 08/436,222. Using that solar collector it has been possible to generate temperatures in excess of 2,000.degree. C.
In general terms, large aperture dishes which concentrate solar energy have many uses requiring temperatures in the range of from 200.degree. C. to over 1,500.degree. C. Among these uses are the cost-effective generation of electricity by the solar thermal path and the powering of many thermochemical and photochemical processes (including the thermochemical conversion of solar energy, solar gasification of hydrocarbon fossil fuel and biomass materials, the production of reduced-pollution or very low pollution liquid and gaseous fuels, and the production of a range of chemicals and other energy-rich products).
When in use, the dish of this type of solar collector must be "tracked" so that it faces the sun throughout the day, in order to focus continuously the incident solar energy on to a receiver (a zone where there is usually a solar energy absorber) which is at the focus of the dish. The tracking requirements are such that any material (solid, liquid or gas) which is to be introduced to the receiver at the energy concentration zone or focal region must be conveyed along a moving path, for the receiver is fixed relative to the dish and thus moves as the dish tracks the sun. It is necessary, therefore, for flexible or rotary joints to be included in the supply line to the receiver. When high temperature and/or high pressure fluids are conveyed in the supply line, such joints are difficult to produce and when certain temperatures and pressures are exceeded, if such joints are available they are costly and detract from the economics of the processes to be powered by the collected solar energy.
Another problem experienced with current solar collector dishes (including the dish depicted in the aforementioned WIPO Publication No. WO 94/11918) is the adequate insulation of the fluid transmission line which conveys ground-based materials to and from the solar receiver. Such a lie or pathway can be of considerable length and the cost penalty associated with its insulation (and with the rotary and flexible joints in this line if it has to weave through a structure of some complexity in order to reach the receiver) is a significant proportion of the cost of the entire solar energy collector system. In addition, significant pumping power may be required to convey the materials to the receiver, which is not readily accessible while moving, and instrumentation for the fluid transport system also poses problems.
A further factor which affects the cost-effectiveness of altitude-azimuth tracking of such solar collector dishes is the dish actuation mechanism (the driving mechanism) which ensures that the collector faces the sun. This requires robust components if it is to be capable of controlling the position of the dish in strong winds.
A collector using a dish which has a conventional aperture shape (circular or hexagonal) and employing altitude-azimuth tracking throughout the day is normally mounted on a tower (so that it can receive solar radiation when the sun is on or slightly above the horizon). The dish may be balanced about its transverse (horizontal) axis of rotation by the addition of a counterweight which is supported on the dish support frame, to make the rotation of the dish about that axis easier to achieve and control. However, counterweights (i) are expensive, (ii) add to the total load being rotated, and (iii) complicate the structural requirements for the collector or antenna. Such a collector will be affected by winds at all times and this means that the strength requirements for the collector structure are increased in locations where high winds are likely to be experienced. In addition, the mechanisms for rotating the dish and their associated hardware require special features to enable some components to be reached (for checking, adjustment or replacement). Those features add to the cost and inconvenience of even routine maintenance.
As persons familiar with dish-tracking arrangements will be aware, if the dish is to be tracked by polar-equatorial movement of the dish, there is a dish transverse (generally east-west) axis about which the dish is rotated to vary its equatorial inclination, and a north-south (polar) axis about which the dish is rotated during the day to track the sun from sunrise to sunset. The structures normally used for polar-equatorial tracking are relatively high above the ground, to allow for rotation of the dish about the polar axis, and thus have to be constructed to withstand high wind loading, and the dish actuation mechanisms must also be robust.
Constructing the reflective surface of a large aperture dish is also a costly exercise. This surface is sometimes approximated by a series of plane mirrors of sufficient number and of such size that the necessary concentrating ratio is achieved. Such mirrors need to be mounted on the dish support frame in a manner which allows each segment of the mirror to concentrate radiation on to the receiver (that is, the dish support frame itself must be of the correct shape to achieve the necessary concentration of the solar energy). Except in the case of very low temperature receivers, the number of plane mirror segments must be large. Thus it is more usual to construct the dish using a lesser number of curved mirrors. When many plane mirror segments or curved mirrors are employed, a significant fraction of the collector cost is incurred in the construction of the reflective surface. Hence considerable developmental effort has gone into producing more cost-effective reflective surfaces during recent years.